Fix bug #3835 Disabling CodeMirror in GUI settings does not actually dis...

Earlier the codemirror.js file was added to the list of default scripts based on the 'CodemirrorEnable' condition when PMA_Header singleton was instantiated. However by the time the singleton is instantiated user preferences are not being merged to the configs loaded from the config file. (instantiated at https://github.com/kasunchathuranga/phpmyadmin/blob/Bug3835/libraries/common.inc.php#L732 and user preferences merged at https://github.com/kasunchathuranga/phpmyadmin/blob/Bug3835/libraries/common.inc.php#L1050). Hence, disabling codemirror from UI did not take effect. As a solution I've moved adding codemirror.js to the getDisplay() method that is called after user prefs are merged to configs loaded from file.